1. Field of the Invention
This invention relates to an analog signal level monitor, and more particularly, to an analog signal level monitor including a means for indicating when, for example, a system A/D converter is operating at the desired input voltage level for input signals with a known distribution function.
2. Description of the Prior Art
When quantizing analog signals by an A/D converter, it is important to set the input analog level at an optimum rms voltage; a level that is too low does not make use of the full resolution capacity of the converter, while a level that is too high will cause the A/D converter to saturate. Once the optimum rms voltage has been determined, a permanent level-monitor to insure operation at this voltage is provided. One such arrangement employs a true-rms voltmeter. However, due to its bulk and expense such a voltmeter is not practical for many applications. As an inexpensive alternative, a diode detector may be placed in the analog path and calibrated for a specific meter reading when the correct input level is applied. One such detector arrangement is disclosed in U.S. Pat. No. 3,809,925 issued to D. Hertz on May 7, 1974. In this arrangement, the detector consists of a number of individual level detectors where each detector includes a tunnel diode buffered from an input signal through a differential amplifier and a hot carrier diode. The differential amplifier is coupled to the analog input source and a reference voltage source through different, relatively high impedance, emitter followers. The hot carrier diode and tunnel diode function to switch from a first quiescent state to a second quiescent state when the level of the analog input signal becomes greater than the desired level established by the reference voltage.
An alternative arrangement is discussed in the article "Input Overload Protection Circuit for A/D Converters" by S. Ganesan in Electronic Engineering, Vol. 50, No. 610, August 1978 at page 8. A protection circuit is discussed which comprises two zener diodes placed back to back. This zener arrangement serves as a double side clipper, and prevents the voltage from exceeding either limit set by the range of the clipper. A potentiometer is connected across the output of the clipper circuit, the middle point of which is connected to the gate of a triac. An LED is used as an indicator lamp to signal when the clipper just starts clipping and the triac starts conducting.
A voltage level sensing circuit comprising a transistor arrangement is disclosed in U.S. Pat. No. 3,400,279 issued to H. A. Grant on Sept. 3, 1968. The Grant device provides the semiconductor circuit arrangement adapted to provide a response only when an input signal applied thereto falls within a predetermined voltage interval. The device comprises both a PNP and an NPN transistor arrangement adapted to produce an output signal which indicates when the level of a DC voltage signal applied as input thereto lies between the two predetermined voltage limits set by the PNP and NPN transistors.
All of the prior art arrangements described hereinabove, however, comprise semiconductor components that are sensitive to drift, demand active analog circuitry and may not possess the flat frequency characteristics desired. The problem remaining in the prior art, then, is to provide a means for monitoring an analog signal level to a device as, for example, an A/D converter, that avoids the use of such sensitive semiconductor components.